[Updated from time to time] LetCode algorithm and library

Always be aware

――Do you have a deep understanding of the technology you have touched on? --Paste the index to make it faster. ⇒ Why? ――People who have a shallow understanding of the concepts that they have touched on in the past will continue to work with a shallow understanding.

--To be able to understand and explain complex concepts

algorithm

Binary search tree (find L or more and R or less in depth-first search)

#Recursive function
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def rangeSumBST(self, root: TreeNode, L: int, R: int) -> int:
        valid_vals = []
        def search(root):
            if root.val >= L and root.val<=R:
                valid_vals.append(root.val)
            if root.left:
                search(root.left)
            if root.right:
                search(root.right)
        search(root)
        return sum(valid_vals)

Standard library

Base conversion

#Decimal number 2,8,Convert to hexadecimal
x = 10
print(bin(x))
print(oct(x))
print(hex(x))

# 2,8,Convert hexadecimal numbers to decimal numbers
print(int('10100', 2))
print(int('24', 8))
print(int('14', 16))

Uppercase / lowercase

s_org = 'pYThon proGramminG laNguAge'
s_org.upper()
# PYTHON PROGRAMMING LANGUAGE
s_org.lower()
# python programming language

zip is used with for statement

class Solution:
    def busyStudent(self, startTime: List[int], endTime: List[int], queryTime: int) -> int:
        cnt = 0
        for m, n in zip(startTime, endTime):
            if m <= queryTime and n >= queryTime:
                cnt += 1
        return cnt

List ascending / descending

l1 = ['d', 'b', 'c', 'a']
l2 = sorted(l1)
l2 = sorted(l1, reverse=True)
print(l2) # ['d', 'c', 'b', 'a']

Determine if the list is empty

a = []
if not a:
 print("The list is empty")

math library

ceiling function

Find the smallest integer greater than or equal to x for a real number x

from math import ceil
c = ceil(r)